(1) Field of the Invention
The present invention relates to a high-speed cell search system for CDMA, and a high-speed search circuit for use in a CDMA (Code Division Multiple Access) communication system having larger interfering wave component power than target signal component power, and more particularly to a high-speed cell search system in which S/N (Signal/Noise) is increased by performing coherent integration over plural signals for search, and identification and screening of long codes are performed by using an orthogonal code.
(2) Description of the Related Art
In a CDMA cellular system, a mobile station must establish synchronization of a spreading code sequence before it establishes synchronization of carrier and clocks. It may take long time to establish the code synchronization, and it is a critical problem for CDMA to shorten this time. Specifically, the conventional CDMA cellular system has the following two problems.
A first problem is as follows. That is, the mobile station performs a cell search for detecting a connection cell on the assumption of an estimation method of initial synchronization of spreading codes.
Conventionally, in an inter-base-station synchronization system which is represented by IS-95, the respective base stations synchronously operate in cooperation with one another on the basis of a predetermined offset amount, and thus each base station 1 can perform the cell search work by using only one type of spreading code even when it moves from one base station to another base station, so that the search time can be shortened.
However, in order to use this method, the base stations are required to be synchronized with one another, and in order to satisfy this requirement, a system for generating a reference synchronous signal to establish external synchronization between the base stations is separately needed.
Therefore, an inter-base-station asynchronous system which needs no external system has been required. However, each base station cannot have one common spreading code because the base stations are asynchronous therebetween. Therefore, the respective base stations have different kinds of spreading codes, and thus all kinds of spreading codes must be searched in a process of acquiring and following the initial synchronization, resulting in increase of the search time.
A second problem is as follows. A method of acquiring synchronization and following synchronization on the basis of an coding gain by a Viterbi processor provides high-quality data on convolution-encoded information. However, at the time when the synchronization of the spreading code sequence is established, the Viterbi processor does not operate because it is before data are read in, and thus the detection is performed in the state that there is no coding gain. Since nothing is started (for example, no carrier synchronization is started) unless the synchronization establishment is completed, it is necessary to perform the detection with high quality even when there is no coding gain. That is, if there is some probability that erroneous detection occurs, the repetition of the search work would occur, resulting increase of the search time.